Poly(vinylphosphonic acid ester) such as poly(dimethyl vinylphosphonate) and poly(vinylphosphonic acid) are being developed as a polymer electrolyte material for fuel cell, a halogen free frame retardant, a metal surface treatment agent, a biocompatible material, a food packaging material, and the like, and analysis of the polymer structure and studies of the polymerization method are actively being conducted.
Poly(vinylphosphonic acid) is, for example, obtained by radical polymerization of vinylphosphonic acid, and it is reported that poly(vinylphosphonic acid) obtained by radical polymerization of vinylphosphonic acid has a large proportion of head-to-head or tail-to-tail linkages and shows low positional regularity (NPL 1).
On the other hand, poly(vinylphosphonic acid) can also be obtained by hydrolyzing in the presence of an acid a poly(vinylphosphonic acid diester) obtained by radical polymerization of a vinylphosphonic acid diester. The poly(vinylphosphonic acid) obtained by the hydrolysis has a large proportion of head-to-tail linkage, and shows higher positional regularity than one obtained by radical polymerization of vinylphosphonic acid (the same as above).
However, in the radical polymerization of a vinylphosphonic acid diester, chain transfer to a phosphorus atom to which alkoxy binds occurs, and therefore molecular weights of the poly(vinylphosphonic acid diester) and the poly(vinylphosphonic acid) obtained by hydrolysis thereof have not been able to be increased.
As a method for obtaining a poly(vinylphosphonic acid diester) having a larger molecular weight, anionic polymerization is used (NPL 2). It is reported that, in the anionic polymerization, a poly(vinylphosphonic acid diester) having a larger molecular weight can be obtained, and in addition, stereoregularity of a poly(vinylphosphonic acid) obtained by hydrolyzing the obtained poly(vinylphosphonic acid diester) is higher compared to one obtained by hydrolyzing the radical polymerization product, and that thermal behavior and solubilities in solvents are also different (the same as above).
Furthermore, as a method capable of controlling the molecular weight, a group transfer polymerization method (GTP) is reported in which a tricyclopentadienyl lanthanoid complex is used as an initiator (NPL 3). This GTP is one of living anionic polymerization methods, and is capable of controlling the molecular weight with the ratio of the monomer and the initiator and obtaining a polymer having a higher molecular weight and lower dispersion.
Incidentally, as the vinylphosphonic acid diester which is a raw material monomer in the poly(vinylphosphonic acid diester) production, dimethyl ester, diethyl ester, diisopropyl ester, and the like are used. Among them, dimethyl ester has a high solubility in water and is suitable for hydrolysis in an aqueous solution. Dimethyl ester is advantageous also in terms of high industrial availability.
However, in the anionic polymerization and GTP, when dimethyl vinylphosphonate is used as a raw material monomer, the produced polymer has low solubility so that the yield of the polymerization is not enhanced, and the molecular weight cannot be increased and also cannot be controlled (NPL 2 and 3 above). Thus, the molecular weights (weight average molecular weights; Mw) of all the polymers produced using dimethyl vinylphosphonate as a monomer have been 50,000 or less, and a poly(dimethyl vinylphosphonate) having a high molecular weight of 60,000 or more has not been obtained, whereby the use thereof is limited. As a result of the above, moreover, all the poly(vinylphosphonic acid)s obtained by hydrolyzing the poly(dimethyl vinylphosphonate) have also had a low molecular weight.
On the other hand, poly(diisopropyl vinylphosphonate) produced by using diisopropyl vinylphosphonate as a monomer, and the like, has low aqueous solubility, and for producing poly(vinylphosphonic acid) from this polymer, it is required to allow trimethylsilyl bromide to react therewith in dichloromethane to convert the ester group to a trimethylsilyl ester and then hydrolyze the resultant in the presence of an acid, and the direct hydrolysis in an aqueous solution has been difficult (NPL 3 above).